The invention relates to a method for functional testing and to a functional test circuit.
Permanent-magnet synchronous machines (PSM) are used, for example, for the drive in electric cars. As in other applications as well, they are used there in the weak-field mode, that is to say the magnetic field is reduced by a field-weakening current. This allows higher rotation speeds to be achieved. In an electric car, a DC battery and a converter between the battery and PSM, for conversion of the single-phase battery voltage to the three-phase motor supply, is also provided by way of example as a drive system for the operation of the PSM.
If the upstream converter fails at high rotation speeds of the PSM, the field-weakening current in consequence fails. As long as the rotation speed remains high, that is to say at least for a short time, the magnets in the motor induce a voltage which is above the normal and desired intermediate-circuit voltage or battery voltage. However, an excessively high voltage is destructive to modern batteries.
One possible design for protection of the battery is to provide one or more diodes between the converter and the battery, and one or more semiconductor switches in parallel with the diode or diodes. In this case, it is highly advantageous for it to be possible to switch the semiconductor switch off at all times.
One possible way to ensure increased reliability is to use redundant semiconductor switches. However, the electrical power loss, which is lost in the semiconductor switches, increases corresponding to the number of additional semiconductor switches used. One possible way to avoid this disadvantage is to test whether the semiconductor switch is functional.